Selector circuit



15, 1946. c. H. SMITH, JR., ETAL I 2,409,229

SELECTOR CIRCUIT Filed June 13, 1945 2 Sheets-She et 1 3mm CARL HARRISON SMITH JR. MILTON L. KUDER Get. 15, 1946.

c. H. SMITH, JR., ETAL 2,409,229

SELECTOR CIRCUIT Filed June 15, 1945 2 Sheets-Sheet 2 I LET-E b VWVJW CARL HARRISON SMITH JR. MILTON L; KUDER Patented Oct. 15, 1946 SELECTOR CIRCUIT Carl Harrison Smith Jr., Arlington, Va., and

Milton L. Kuder, Washington, D. 0. Application June 13, 1945, Serial No. 599,287

(Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 0. G. 757) 7 Claims.

This invention relates in general to a pulse transmission system and in particular to a means for producing a group of pulse signals wherein the number and the relative time occurrence of the pulse signals comprising the group are selectively determined.

In an application filed May 11, 1945, Serial No. 593,174, by E. H. Krause et al., a pulse transmission system is disclosed in which a group of pulse signals is transmitted over a given time interval. The number and the relative time occurrence of the pulses comprising the group are made controllable. In this system a time base made up of a number of successive unit time intervals is generated at the transmitter and comprises a time pattern from which the group to be transmitted is formed. In any given transmission a pulse signal is always transmitted coincident with the first unit time interval in the time base and at such other unit time intervals as may be desired. At the receiving system a second timebase generating device and a series of amplifying channels are provided and arranged so that each unit time interval generated by the time base device renders a different one of the amplifying channels operative. In operation the first pulse received from the transmitter starts the time base at the receiver then each pulse signal arriving after the first pulse signal is made to pass through a different amplifying channel according to its time relationship with respect to the first pulse in the group. In this manner any given pulse signal occupying a definite time position relative to the start of the interval of transmission can be made to pass through a particular amplifying channel and can therefore be used to perform a definite function at the receiving system.

It is an object of this invention to provide a means for producing a group of pulse signals wherein the number and relative time occurrence of the pulse signals are selectively determined.

It is another object of this invention to provide a means for alternately producing two dis tinct groups of pulse signals wherein the disposition of the pulse signals in each group is selectively determined.

Other objects and features of the present invention will become apparent upon a careful con sideration of the following detailed description when taken together with the accompanying drawings.

Fig. l is a circuit diagram of one embodiment of the invention, and Figs. 2 and 3 show a series of wave forms illustrative of the operation of the circuit shown in Fig. 1.

According to the invention, a time base generator, which may comprise a delay line energized by a pulse generator or any other suitable arrangement as hereinafter described, is provided and arranged to cooperate with a series of pulse selector vacuum tubes. Associated with each of the tubes is a separate switching means which may be operated to effectively withdraw any one or number of tubes from the series. The vacuum tubes and the time base generator are so interconnected that when the latter is set into operation it functions to successively render con'. ducting those tubes left in the series by the switching apparatus. Connected to the tubes in the series, but, not shown for purposes of simplification, is a pulse transmitter which is held under control of the pulse selector tubes and ar-' ranged to emit a pulse signal each time one of the tubes in the series becomes conducting.

With particular reference to Fig. l a detailed circuit diagram of the invention is shown. As indicated in the figure, the time base generator comprises a pulse generator l0, and a scale-offive counter chain, including stages ll, :3. Tubes l4 through l8 represent the pulse selector tubes with which the time base generator is to operate, while tube 24 is representative of a coincidence tube which is connected in common to the pulse selector tubes by way of abiasing tube 23. Also, a pulse transmitter, not shown, is connected to the output terminal 55. As will hereinafter be described in detail, the pulse generator ll may be a multivibrator or any other suitable pulse-generating device, such'as that shown in the aforementioned patent application, which may be started and stopped either automatically or manually, such as by the off-on switch H, for example. The primary function of this pulse generator is to provide a source for driving the counter chain, comprising stages H, 12, and i3, through a complete cycle of operation and at the same time to provide transmitter key;

ing pulses to the first control grid 35 of the coincidence tube 24. By-int erconnection of the control grid of each of the selector tubes l4 through I 8 to predetermined points in the counter chain as hereinafter described in detail and in accordance with the principles taught in the patent application of E. H. Krause Serial No. 582,964, filed March 15, 1945, the counter circuit is made to successively render each of the selector tubes conducting, as far as their grids are concerned, during a cycle of operation. When any of the selector tubes conducts, a voltage drop appears across their common plate load resistance 29.

The presence or absence of this voltage drop results respectively in either non-conduction or conduction of the biasing tube 23, which by way of its connection to the second control grid 34 of the coincidence tube 24, renders the latter either sensitive or insensitive, respectively, to positive voltage pulses applied to its first control grid 35 by the pulse generator In. Any one of the selector tubes I4 through I3 can be prevented from going into conduction in response to the action of the counter chain by opening the cathode circuit of the respective selector tube. Therefore to selectively suppress or produce output pulses at the terminal 55 of the coincidence tube 24, a number of multi-position switches numbered I9 through 22 are inserted in the respective cathode circuits of the selector tubes I-5 through I8. In the zero position of these switches the cathode circuit is complete to ground and the selector tube is capable of conduction in response to the action of the counter chain. In the No. 3 position of the switches the cathode circuit is open and the corresponding selector tube will be incapable of conduction regardless of its grid bias voltage as provided by the counter circuit. Consequently, during any given cycle of counter operation the appearance or absence of a transmitter keying signal at the output terminal 55 depends upon the setting of the cathode switches l9 through 22.

In the typical counter chain shown here all of the counter stages II, I2, and I3 are similar in structure, also the grid biasing resistances and plate resistances employed at corresponding points in each half of each counter stage are of similar resistive value, resistances 43, 46 similar to 4|, 45 and so on. Therefore for purposes of simplification, only the first counter stage II will be described in detail. In this stage the multigrid' tubes Ila and Ill) represent the vacuum tube components of a conventional scale-of-two counter circuit. The first control grid 38 of tube Ila is connected through the shunt combination of resistance 43 and capacitance 44 to the plate of tube Ilb and also, through the grid return resistance 46 to a source of C: potential. Likewise, the first control grid 39 of tube III) is connected through the shunt combination of resistance'4l and capacitance 42 to the plate of tube I la and also, through the grid return resistance 45 to a source of potential. The circuit is thus made regenerative and only one of the tubes will conduct at any instant. To change the conduction condition of either tube a negative signal is applied in parallel to the second control grids 36 and 31 of both tubes. As the conduction state of the circuit changes, the first control grid of the tube passing from conduction to nonconduction will vary from about zero potential to a negative potential determined by the voltage from the positive potential at the plate of III) to the negative supply potential across resistances 43, 46. The input to the counter circuit as obtained from the pulse generator IE] is applied in parallel to the control grids 36 and 31 through a coupling circuit comprising capacitance 6B and resistance 6|. In one case this circuit may constitute a short time-constant compared to the time duration of the applied pulse and therefore functionto, produce an abrupt positive pulse in response to the positive-going edge of each of the applied pulses and an abrupt negative pulse in response to the negative-going edge of each of the applied pulses. The input to the second counter circuit I2 is taken-from a tap on the plate load divider action 6 resistance of the second tube llb in the first counter stage and is applied in parallel through a second short-time-constant circuit similar to capacitance 60 and resistance SI to the second control grids of tubes lie and I2!) of the second counter stage I2. Finally, the input to the last counter stage I3 is taken from a tap on the plate load resistance of the second tube IZb in the second counter stage and applied in parallel through 10 another short-time-constant circuit similar to capacitance 6i) and resistance 51 to the second control grids of tubes I3a and I31) of the final counter stage I3. As hereinafter described, the quiescent or zero state for the counter chain exists when tubes I la, I2a, and H51) are conducting and tubes Ilb, I21), and I3a, are non-conducting. In tracing a complete cycle of counter operation, the first input pulse applied to counter stage II renders tubes I'Ib, I21), and I311 conducting and tubes Ila, 2O I2a, and l3b non-conducting The second input pulse renders tubes Ila, I21) and I3a conducting. A third pulse from the pulse generator will render tubes III), In and I3a conducting. The fourth input pulse renders Ila, I2a and I3a conducting. 25 The fifth negative input pulse would normally render tubes I lb, I21) and I3?) conducting but due to a feedback pulse obtained from the plate of tube I31) and applied through capacitance 58 to the suppressor grids59 and 63 of tubes Ill) and I21) respectively, the tubes Ila, I2a and I3b will be conducting, and tubes IIb, I21) and I3a non-conducting. It will be noted that this state is identical to that which existed when the counter was in the zero or quiescent state. Thus the counter 35 chain is of a scale-of-five variety.

In the selector circuit the control grid of each tube I4 through I8 is connected to a separate junction point I08, 260, 300, 400, and 500. Each of these junction points is in turn connected through three separate resistances to the first ccntrolgrid of one of the tubes in each of the counter circuits II, I2 and I3. This connection is typified at the first junction point I00 by resistances 41, 48 and 49. In this relation it should be noted that all the resistances at each junction point are equal in size and are current limiting in function. At the first junction point I00 current limiting resistance 41 is connected to the first control grid of tube Ila and resistance 48 to 50 the first control grid of tube I2a and the third resistance 49 to the first control grid of tube I3b.

As abovementioned, the first control grid of any tube in the counter circuit varies from zero potential to a potential determined by voltage division across resistances 43, 46 as that tube passes from conduction to non-conduction. Thus, when the counter is in the zero or quiescent state wherein the tubes I la, IZa and I3!) are conducting, the potential at the junction point I00 will be zero and theselector tube M will conduct since its cathode is returned directly to ground. If each of the resistance connections of the remaining junction points 200, 300, 400 and 500 are traced it will be found that in the zero state of the count- 65 er, a minimum of at least one resistance at each of the other junction points is returned to a grid in the counter circuit which is at a negative potential. v The minimum negative voltage existing at any one of these junction points due 7 to the voltage divider action of the current limiting resistances is therefore at least of the negative voltage existing at the junction of resistances 43, 45 when tube llb is conducting. Thus, if the negative potential is made large relative to 7 the cut-off bias for the selector tubes the grids of the remaining tubes in the selector circuit will be biased below cut-off. Then as the counter circult is driven into its No. 1 state where tubes I lb, !2b and 13a are conducting and tubes Ha, l2a and I3?) are non-conducting, it will be observed that the voltage at the junction point 2 08 will now be at zero while the voltage existing at the other junction points I99, 300, 403 and 500 will be sufficiently negative to hold the other selector tubes below cutoff. Tube l5 will now be able to conduct as far as its grid is concerned. In the second state of the counter circuit tube I6 will be the only tube in the selector capable of conducting and so on down the circuit, each selector tube rendered capable of conducting in its proper order with each change in state of the counter circuit.

The coincidence tube 24 is preferably of the pentagrid type having two control grids 34 and 35. The first control grid 35 provides a means by which the pulse signals from the pulse generator Ill may be applied to it, while the second control grid 34 provides a mean by which the coincidence tube may be blocked and unblocked, so as to speak, by the joint action of the counter circuit and the selector circuit to thereby controllably suppress or produce an output pulse at the terminal 55. The first control grid 35 is returned through resistance 62 to the juncture point of the cathode resistances 32 and 33. As a result of the voltage dividing action of these two resistances, and the cathode bias voltage 57, the coincidence tube is held unresponsive to negative signals applied to this grid. The second control grid 34 is directly connected to the plate of the biasing amplifier 23 which in turn is grid driven from the common plate load resistance 29 of the selector tubes. During the time when all of the selector tubes are non-conducting, the grid of tube 23 will rise above cathode potential until limiting by resistance 29 occurs. This results in heavy conduction by the biasing tube 23 producing a large voltage drop across its plate load resistance. The plate potential of tube 23 willthen become only slightly higher than the positive cathode potential 56. This latter potential is made some suitable value less than the cathode potential 5? on the coincidence tube so that in this condition the bias on the second control grid 34 is such as to hold the coincidence tube blocked or non-conducting regardless of the signal applied to the first control grid 35. When, however, any one of the selector tubes becomes conducting, a voltage drop appears across their common plate load resistance 29. This drop is sufficient to bias tube 23 beyond cut-off potential. The plate voltage of this tube then rises to its plate supply voltage 64. This supply voltage is made equal to the cathode bias voltage 51 of the coincidence tube and the second control grid 36 of the latter is therefore possessed with zero bias and the coincidence tube is ready to respond to any positive signal applied to its first control grid 35.

For purposes of illustration assume that the cathode switches 26 and 21 in the selector tubes 3 and i 3 are et in the zero position while switches 59 and 22 in the selector tubes and #8 are set in the No. 3 position. With this assumption and the further assumption that the counter circuit is initially in its zero or quiescent state, the circuit will now be described with reference to the wave forms shown in Fig. 2. Waveform a i representative of the output signal from the pulse generator Hi. This output wave is applied in parallel to the short time-constant circuit comprising capacitance 69 and resistance 6| to the input of the counter circuit and also through the short time-constant circuit comprising capacitance 60 and resistance 62 to the first control 5 grid of the coincidence tube 24. Waveform b is the result of the action of the foregoing short time-constantcircuits and is representative of the actual signal voltage applied to the input of the counter circuit and to the first control grid 35 of the coincidence tube 24. Since the counter circuit was initially assumed as being in the zero or quiescent state with the selector tube l4 conducting and therefore with the bias removed from the second control grid 35 of the coincidence tube 2:! and since the first pulse in waveform b is positive and does not eifect a change in the counter state, this puls will appear at the output terminal 55 as a negative pulse such as that shown in waveform c. The next pulse in waveform b is negative and will therefore be blocked from the output terminal 55 of the coincidence tube 24 by the action of the cathode bias 5! but will, however, produce the number one state in the counter circuit wherein tubes lib, I21), and l3a are conducting and tubes Ila, [2a and I32) are non-conducting. This action will render the selector tube I5, as far as its control grid is concerned, conductin but since its cathode switch I9 was assumed adjusted to the No. 3 position its cathode circuit will not be complete and therefore the tube will not conduct. In this case, the second control grid 34 of the coincidence tube 24 will be biased negative and therefore the second positive pulse will not produce an output at the terminal The second negative pulse operates to produce the second state in the counter circuit with tube i6 conducting as far as its grid is concerned and since this is one of the tubes whose cathode switch 20 was set in the zero position, the tube will conduct and the bias will be removed from the second control grid 34 of the coincidence tube with the result that the third positive pulse causes conduction by tube 24 producing, with negligible delay, a negative pulse at the output terminal 55. Thus the combined actions of the counter circuit and the selector circuit operate to produce at the output terminal 55 a pulse group formation such as that shown by waveform c. In this case the first, third and fourth pulses were produced at the output terminal while the second and fifth were suppressed.

From the foregoing it was seen that whether or not a pulse appears at the output terminal 55 and is therefore transmitted depends upon the 55 setting of the cathode switch arrangement in the selector circuit. In some instances it may be desired to transmit for a given interval of time one group of pulse signals and then to transmit for another interval of time a second group of Pulse signals having a different formation. To

accomplish the foregoing, a multivibrator comprising tubes 25 and 26 and a pair of diodes 2! and 28 are disposed within the cathode circuits of the selector tubes and are arranged so as to produce cathode switching in a predetermined manner. The multivibrator is preferably a symmetrical free-running type having a period much longer than the time required to make several successive transmissions during the conduction period by either of the tubes. When tube 25 is conducting, tube 26 will be non-conducting. During this time the voltage at the plate of tube 25 will be near that of its cathode which is biased negative as indicated at 18, while the plate of the non-conducting tube will be substantially 3+.

As arranged, the cathodes of the associated diodes 2! and 28 are connected through the respective current limiting resistances 30 and 3! to the plates of tubes 28 and 25. Thus when tube 26 is conducting its plate potentia1 is nearly that of source which is sufliciently negative to permit conduction by the associated diode 21. At the same instant tube 25 is non-conducting so that its plate potential will be at B+ with the result that the associated diode 28 is also held non-conducting. The diode that conducts appears essentially as a short circuit to ground while the non-conducting diode appears as an open circuit. Thus by Connecting the cathode, for example, of diode 2"! to all the Number 1 positions in parallel of the switches 5% through 22, and the cathode of diode 23 in parallel to all the No. 2 positions in the switches, the multivibrator action can make the No. 1 and 2 positions of the switches appear alternately as open and closed circuits respectively.

With reference to the waveforms shown in Fig. 3 the operation of the circuit including the action of the multivibrator and the associated diodes 21' and 28 will now be described. In Fig. 3, waveform a is representative of the alternating positive and negative voltage pulses obtained from the pulse generator ill and applied in parallel to both the counter circuit and the coincidence tube 24. For purposes of illustration let it be assumed that the cathode switch 22 is set in the #2 position and the remaining cathode switches i9, 25 and 2i are set in the #1 position and that tube 25 in the multivibrator circuit is conducting. During this condition diode 28 appears as a short circuit and all the #2 positions in the switches l9 through 22 appear at ground while diode 2'1 and consequently all the Number 1 positions in the switch appear as an open circuit. In this case only the first pulse and the fiith pulse will be transmitted as represented by waveform I). Then, after a given number of such transmissions the multivibrator changes states wherein tube 25 is conducting and tube 25 is non-conducting. During this time diode 21 will conduct and the #1 terminals in the cathode switches will be substantially at ground potential while all the Number 2 positions appear as open circuits. The resulting transmission is that shown by Waveform 0. In this case the first, second, third and fourth pulses produce transmission, the fifth pulse is suppressed.

Although we have shown and described the use of a scalc-of-five circuit as a time base generator it must be understood that if a counter circult is to be used, one of a larger scale and a selector circuit having a correspondingly larger number of tubes may be employed without departing from the spirit of the invention. It is also to be understood that selection switches may be inserted in all of the selector tubes rather than all except the first tube as shown in the specific embodiment. Furthermore, it is not necessary to restrict the operation to a pulse generator providing alternate peaked positive and negative pulses to the counter circuit and the coincidence tube 24. For example, a series of pulses of negative polarity may be used to trip the counter circuit while an inverter tube can supply positive pulses to the second control grid 3%: of the coincidence tube 24 or the negative pulses may be applied directly to the cathode of tube 2d. Despite the fact that the counter pulses and the transmitter keying pulses occur simultanecusly the system is still operative because a certain amount of time delay is inherent in the operation of the selector tubes. This delay is primarily due to the large resistances employed at 41, 48 and. 49 and other similar positions which impose a finite time delay when charging or discharging the stray capacitances associated with these points. In addition this delay may be accentuated by actually inserting physical capacitances at these points. Therefore this invention is not to be limited to the embodiment shown except insofar as is necessitated by the spirit of the prior art and the scope of the appended claims.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. A means for producing a group of pulses wherein the number and relative time occurrence of the pulse signals in said group are selectively determined, comprising a means generating a time base which is comprised of a finite number of voltage pulses, each of which is predeterminately spaced in time, a plurality of vacuum tubes, means associating each of said tubes with said first named means in such a manner as to be successively rendered conducting thereby, separate means associated with each of said tubes and arranged to render any one or number of said tubes unresponsive to the action of said first named means, a coincidence tube, means connecting the output from said first named means to said coincidence tube, and means connecting said coincidence tube in common with said plurality of tubes in such a manner that the pulse output from said first named means will appear at the output of said coincidence tube only during the time when one of said plurality of tubes is conducting.

2. A means for producing a group of pulses wherein the number and relative time occurrence of the pulse signals in said group are selectively determined, comprising a means generating a time base which is comprised of a finite number of voltage pulses each of which is predeterminately spaced in time, a plurality of vacuum tubes, means associating each of said tubes with said first named means in such a manner as to be successively rendered conducting thereby, separate switching means associated with each of said tubes and arranged to render any one or number of said tubes unresponsive to the action of said first named means, a coincidence tube, means feeding the output from said first named means to said coincidence tube, and means connecting said coincidence tube in common with said plurality of tubes in such a manner that the pulse output from said first named mean will appear at the output of said coincidence tube only during the time when one of said plurality of tubes is conducting.

3. A means for producing a group of pulses wherein the number and the relative time occurrence of the pulse signals in the group are selectively determined, comprising a plurality of the scale-of-two type of counter stages connected in cascade so as to form a scale-of-N counter circuit, a plurality of vacuum tubes each of which is connected to said counter circuit in such a manner as to be successively rendered conductingthereby as said counter circuit passes through a cycle of operation, separate switching means connected to each of said tubes for rendering any one or number of said tubes unresponsive to the action of said counter circuit, a pulse generator arranged to produce a series of voltage pulses, means applying the output of said pulse generator to said counter circuit, a coincidence tube, means also applying the output from said pulse generator to said coincidence tube, and means connecting said coincidence tube in common with said plurality of tubes in such a. manner that the pulse output from said pulse generator will appear at the output of said coincidence tube only during the time when one of said plurality of tubes is conducting.

4. A means for producing a group of pulses wherein the number and the relative time occurrence of the pulse signals in the group are selectively determined, comprising a plurality of the scale-of-two type of counter stages connected in cascade so as to form a scale-of-N counter circuit, a plurality of vacuum tubes each of which is connected to said counter circuit in such a manner as to be successively rendered conducting thereby as said counter circuit passes through a cycle of operation, separate switching means connected to each of said tubes for rendering any one or number of said tubes unresponsive to the action of said counter circuit, a pulse generator arranged to produce a series of voltage pulses, means applying the output of said pulse generator to said counter circuit, a Coincidence tube having at least two control electrodes, means applying the output from said pulse generator to one of said control electrodes in said coincidence tube, and means connecting the other control electrode of said coincidence tube in common to said plurality of tubes in such a manner that the pulse output from said pulse generator will appear at the out-put of said coincidence tube only during the time when one of said plurality of tubes is conducting.

5. A means for alternately producing two distinct groups of pulses wherein the number and the relative time occurrence of the pulse signals in each group are selectively determined, comprising a plurality of the scale-of-two type of counter stages connected in cascade so as to form a scale-of-N counter circuit, a plurality of vacuum tubes each of which is connected to said counter circuit in such a manner as to be successively rendered conducting thereby as said counter circuit passes through a cycle of operation, an electronic switching means, said switching means alternating back and forth between two electrical states, separate manual switching means connected to each of said tubes and to said 1' electronic switching means, said manual switching means being arranged so that a first group of said tubes becomes unresponsive to the action of said counter circuit during one electrical state of said electronic switch and a second group of said tubes becomes unresponsive to the action of said counter circuit during the second electrical state of said electronic switch, a pulse generator arranged to produce a series of voltage pulses, means applying the output from said pulse generator to said counter circuit, a coincidence tube, means also applying the output from said pulse generator to said coincidence tube, and means connecting said coincidence tube in common with said plurality of tubes in such a manner that the pulse output from said pulse generator will appear at the output of said coincidence tube only during the time when one of said plurality of tubes is conducting.

6. A means for alternately producing two distinct groups of pulses wherein the number and the relative time occurrence of the pulse signals in each group are selectively determined, comprising a plurality of the scale-of-two type of counter stages connected in cascade so as to form a scale-of-N counter circuit, a plurality of vacuum tubes each of which is connected to said counter circuit in such a manner as to be successively rendered conducting thereby as said counter circuit passes through a cycle of operation, an electronic switching means, said switching means alternating back and forth between two electrical states, separate manual switching means connected to each of said selector tubes and to said electronic switching means, said manual switching means being arranged so that a first group of said tubes becomes unresponsive to the action of said counter circuit during one electrical state of said electronic switch and a second group of said tubes becomes unresponsive to said counter circuit during the second electrical state of said electronic switch, a pulse generator arranged to produce a series of voltage pulses, means applying the output of said pulse generator to said counter circuit, a coincidence tube having at least two control electrodes, means applying the output from said pulse generator to one of said control electrodes in said coincidence tube, and means connecting the other control electrode in common to said plurality of tubes in such a manner that the pulse output from said pulse generator will only appear at the output of said coincidence tube during the time when one of said plurality of tubes is conducting.

'7. A means for producing a group of pulses wherein the number and relative time occurrence of the pulse signals in said group are selectively determined, comprising a means generating a time base which is comprised of a finite number of voltage pulses each of which is predeterminately spaced in time, a plurality of vacuum tubes, counter means interconnecting each of said tubes to said first named means whereby each of said tubes is caused to be successively rendered conducting as said counter means is driven through a cycle of operation, separate switching means associated with each of said tubes and arranged to render any one or number of said tubes unresponsive to the action of said first named means, a coincidence tube, means feeding the output from said first named means to said coincidence tube, and means connecting said coincidence tube in common with said plurality of tubes in such a manner that the pulse output from said first named means will appear at the output of said coincidence tube only during the time when one of said plurality of tubes is conducting.

CARL HARRISON SMITH, JR. MILTON L. KUDER. 

